KR101399140B1 - Correction of quadrature errors - Google Patents

Abstract

The transmission chain includes a correction network 1a for correcting frequency-dependent quadrature errors between in-phase and quadrature signal paths for the transmission of input signals. The correction network includes an in-phase input port, a quadrature input port, an in-phase output port, and a quadrature output port, each input port connected to a respective output port by a digital filter network, And means for constructing the values of the set of filter tap coefficients. The input signals are upconverted and a portion of the upconverted signal is combined into a quadrature downconverter 21. The controller 22 compares the downconverted signal with the input signal to determine the error signal and corrects the frequency dependent quadrature error by modifying the set of values of the filter tap coefficients based on the error signal and the input signal.

Description

{CORRECTION OF QUADRATURE ERRORS}

The present invention relates to the correction of quadrature errors associated with digital communication systems, and more particularly to corrections in a wireless transmission chain having both direct converters and downconverters.

In the case of a communication system, it is common for the signal to be transmitted to have a digital part to be processed before transmission and, furthermore, a received digital signal to be processed after reception. The processing in the digital parts is typically performed in the baseband, i.e. in the frequency band of the signal before any offsets for transmission at the carrier frequency; In general, the baseband signals include a zero frequency component, i.e., a direct current (DC) component. It is common that the baseband signal is represented by an in-phase (I) and a quadrature (Q) portion, a complex representation. The processing may include procedures such as filtering, modulation demodulation coding and decoding. It is generally necessary to convert signals for transmission and reception between analog domains, and in the case of wireless systems it is necessary to convert between signals and the appropriate radio frequency.

There are various ways to convert digital signals between baseband and radio frequency. One way is to upconvert in the digital domain so that the complex baseband signals are multiplied, or mixed, by a digital local oscillator to produce an output at a higher frequency, often referred to as an intermediate frequency (IF) Can be converted to an analog domain by a digital-to-analog converter. The intermediate frequency signal is a signal only of a real component, not a complex component. The analog signal can then be further frequency switched to the appropriate frequency for transmission. Likewise, upon reception, the signals are converted from analog to digital domain at an intermediate frequency higher than the baseband, and then mixed digitally with in-phase and quadrature baseband signals. It is an advantage of this method that the conversion from the baseband complex signal to the intermediate frequency signal and vice versa are performed digitally and thus are not subject to analog errors that can cause differences between the in-phase and quadrature channels. However, digital-to-analog converters and analog-to-digital converters have the drawback that they must operate at higher frequencies than the baseband in order to convert intermediate frequency signals. Operating these components at higher frequencies results in higher components and potentially lower performance in terms of resolution than lower frequency digital-to-analog converters and analog-to-digital converters.

Another way of converting digital signals from baseband to radio frequency and from radio frequency to baseband is generally referred to as direct conversion. In a direct conversion architecture, baseband phase and quadrature signals are converted to analog form at baseband or from analog form. At the time of transmission, the analog driver phase and quadrature signals are then upconverted in the analog domain by an analog quadrature mixer. Preferably, the upconversion relates to a radio frequency transmission frequency in one step, and consequently the use of an intermediate frequency is not required. Likewise, upon reception, the conversion of the received radio frequency signals is preferably done directly with the in-phase and quadrature baseband analog signals, and then converted for the digital domain. The advantage of the direct conversion method is that suitable digital-to-analog and analog-to-digital converters are less expensive and can have higher performance in terms of resolution. Also, omitting the intermediate frequency step can lead to cost savings because less components are needed. However, there is a potential drawback that in-phase and quadrature signal paths include analog components such as filters that are subject to variations in component values within tolerances, so that the analog properties of the in-phase and quadrature signal paths Depending on and depending on temperature.

Errors that cause degradation in fully orthogonal in-phase and quadrature channels present in the digital domain are known as quadrature errors, or IQ errors. In particular, it can be a problem if there is a differential error between the in-phase and quadrature channels. Differential errors between the in-phase and quadrature channels may e.g. be such that spurious components are generated at the transmitter and a spurious response is generated at the receiver. In particular, a spurious response may be generated in the sideband opposite to the intended one; For example, if the signal component is intended to be at a higher frequency than the local oscillator signal, a differential error between the in-phase and quadrature components may result in a spurious component appearing at a frequency lower than the frequency of the local oscillator signal.

Quadrature errors typically include voltage offset, i.e., DC offset, differential gain characteristics between in-phase and quadrature signal paths, and phase errors between in-phase and quadrature signal paths. An existing quadrature correction network 4 for correcting such quadrature errors is shown in Fig. 1; Gain correction blocks Igain 5a and Qgain 5b are shown and a block 12 labeled IQ phase for correction of phase errors between in-phase and quadrature paths is shown and block I DC Offset 24a and Q DC Offset 24b are shown.

However, quadrature errors in upconversion and downconversion, and in particular differential quadrature errors, may be frequency dependent within the baseband. For example, analog filtering can add these errors to the anti-aliasing filter in particular, due to variations in analog component values and temperature variations within the component tolerance limits. Existing correction networks can not correct these errors.

The present invention solves this disadvantage.

According to a first embodiment of the present invention, a method of controlling a transmit chain is provided. The transmission chain includes a correction network, a quadrature upconverter and a quadrature downconverter, the correction network corrects a frequency dependent quadrature error between the transmission characteristics of the in-phase signal path and the transmission characteristics of the quadrature signal path in the quadrature up- Wherein the quadrature upconverter is for use in upconversion of the input signal to the correction network and the quadrature downconverter is for use in downconverting the received signals and the upconverter is for output coupled to the downconverter, , The correction network may be configured via a set of values of the filter tap coefficients, and the input signal includes frequency components. In one embodiment, the method includes coupling an output signal from an output of the upconverter to a quadrature downconverter; Downconverting the combined signal using a downconverter; Comparing the downconverted signal with an input signal; And modifying values of the filter tap coefficients based on the comparison, thereby correcting the frequency dependent quadrature error by a correction applied to the frequency component, the correction being dependent on the frequency of the frequency component.

The advantage of updating the set of values of the filter tap coefficients based on the comparison of the downconverted signal and the input signal to control the transmit chain is that accurate control of the correction network can be achieved, Characteristics can be achieved.

In one configuration, a downconverted signal is compared to an input signal to determine an error signal; This error signal is then used in conjunction with the input signal to modify the set of values of the filter tap coefficients. A training algorithm can then be used to update the values of the filter tap coefficients based on the error signal and the input signal; The training algorithm is advantageous because it provides an effective way of updating the values of the filter tap coefficients.

Preferably, the method further comprises providing a local oscillator signal generated by the local oscillator signal source to the upconverter and the downconverter, wherein the local oscillator is configured to receive the local oscillator signal from the local oscillator signal input to the upconverter and the downconverter, Operating state; And a second operating state configured to apply a phase shift to an input signal to the upconverter or downconverter when the local oscillator is operating in a first operating state; Comparing the downconverted signal to an input signal for each operating state to determine an error signal; Determining an intermediate set of values of filter tap coefficients based on an error signal and an input signal for each operating state; And updating the current set of values of the filter tap coefficients based on the vector set of the intermediate set of values and the current set of values to generate a set of updated values.

As a result, the correction network can be controlled to correct for quadrature errors in the upconverter even in the presence of quadrature errors in the downconverter.

Preferably, the downconverter operates in conjunction with a post-correction network configured to correct frequency dependent quadrature errors between in-phase and quadrature transmission paths in a quadrature downconverter, and the downconverted signal has a frequency component Wherein the post-correction network comprises configuration means for constructing values of a set of post-correcting filter tap coefficients and a set of post-corrective filter tap coefficients, the method comprising generating intermediate sets of filter tap coefficients, Updating the current values of the post corrective filter tap coefficients based on the vector combination of the post corrective filter set to generate a set of updated post corrective filter coefficients; And controlling the post-correction network using an updated post-corrector set of values, wherein the correction applied to each frequency component dependent on the frequency of the frequency component causes the frequency dependent quadrature in the quadrature down- Correct the runner error. This provides a means for controlling the frequency dependent correction network for the upconverter and the frequency dependent correction network for the downconverter. Moreover, the frequency dependent correction of the downconverter can be used as an input to a predistortion controller for the power amplifier, thereby improving the operation of the predistortion controller.

The foregoing functions may be implemented in software, or computer readable code, on a computer readable medium for use in controlling a correction network, an upconverter, and a downconverter in the manner described above.

According to a second aspect of the present invention there is provided a transmit chain, comprising:

A correction network for correcting frequency dependent quadrature errors between in-phase and quadrature signal paths for transmission of an input signal, the correction network comprising an in-phase input port, a quadrature input port, an in-phase output port and a quadrature output port Each input port coupled to a respective output port by a digital filter network and the digital filter network comprising configuration means for configuring values of a set of filter tap coefficients and a set of filter tap coefficients; And a quadrature upconverter for upconverting the input signal; A coupler for receiving a portion of the upconverted input signal; And a quadrature downconverter for downconverting the signal received by the coupler; Compare the downconverted signal with an input signal to determine an error signal; Modifying the set of values of the filter tap coefficients based on the error signal and the input signal and using the updated set of values to control the correction network so that by correction applied to each frequency component according to the frequency of the frequency component And a controller adapted to correct frequency dependent quadrature errors.

According to a further aspect of the present invention, there is provided a correction network according to claim 1.

More specifically, in accordance with an aspect, there is provided a correction network for correcting frequency dependent quadrature errors between a transmission characteristic of an in-phase signal path and a transmission characteristic of a quadrature signal path, wherein the quadrature signal path is a quadrature signal path, Wherein the correction network comprises an in-phase input port, a quadrature input port, an in-phase output port, and a quadrature output port, each input port connected to each output port by a digital filter network, The digital filter network includes configuration means for configuring values of a set of filter tap coefficients and a set of filter tap coefficients.

The advantage of connecting each input port to each output port by means of a digital filter network comprising a set of filter tap coefficients and constituent means for constructing the values of this set of filter tap coefficients is for example a quadrature upconverter or a downconverter The frequency dependence quadrature impairment can be corrected by appropriate control of the coefficients due to the analog components.

In one embodiment, the digital filter network includes a first digital filter that connects an in-phase input port to an in-phase output port; A second digital filter connecting the in-phase input port to the quadrature output port; A third digital filter connecting the quadrature input port to the inphase output port; And a fourth digital filter connecting the quadrature input port to the quadrature output port, each digital filter having a respective set of filter tap coefficients and respective configuration means for constructing values of the set of respective filter tap coefficients .

Each digital filter can be implemented with a finite impulse response filter, which is effective because the finite impulse response filter can be controlled by suitably selected coefficients to provide a good approximation of the frequency characteristics of the quadrature impairment.

Alternatively, each digital filter can be implemented in a polynomial structure based on the Volterra series, which is effective because it provides highly desirable cancellation of quadrature impairment components.

Figure 1 illustrates a conventional quadrature correction network;
FIG. 2 is a frequency dependent pre-correction and frequency dependent post-correction controlled by comparing a downconverted signal with an input signal in accordance with an embodiment of the present invention; FIG.
3 illustrates a frequency dependent correction network in accordance with an embodiment of the present invention.
Figure 4 illustrates a frequency dependent correction network in accordance with an embodiment of the present invention prior to typical network damage as an example of operation of an embodiment of the present invention.
5 illustrates a digital filter component of a frequency dependent correction network in accordance with an embodiment of the present invention.
Figure 6 illustrates a controller for a frequency dependent pre-correction network and a frequency dependent post correcting network in accordance with an embodiment of the present invention.
7 is a diagram illustrating frequency dependent pre-correction and frequency dependent post-correction according to an embodiment of the present invention.
8 illustrates a frequency dependent pre-correction according to an embodiment of the present invention.
9 illustrates details of a controller for a frequency dependent pre-correction network in accordance with an embodiment of the present invention.
Figure 10 shows a conventional pre-correction and post-correction network controlled by optimization of prediction properties of a down-converted signal.
11 is a diagram illustrating frequency dependent pre-correction and frequency dependent post-correction in accordance with a further embodiment of the present invention.

In general, the present invention relates to a method and apparatus for correcting quadrature errors in a communication system.

By way of example, an embodiment of the invention is a transmission chain of a wireless system, in which a digital signal is upconverted in a direct conversion transmission chain and a sample of the transmitted signal is downconverted in a direct conversion receiver for reception by an observing receiver Is described in the context of a series of components. The observing receiver may be used for control of the predistortion function applied to the digital signal prior to upconversion to pre-correct the nonlinear response of the power amplifier. However, this example is for illustrative purposes only and the invention is not limited to use for a system or for use in a wireless system associated with predistortion of a nonlinear amplifier.

Fig. 2 shows a first embodiment of the present invention. A digital baseband signal having an in-phase component 2i and a quadrature component 2q is input to the pre-corrector 1a and then to a direct conversion IQ up- The IQ upconverter 17 includes a digital-to-analog converter for the in-phase and quadrature components, and the resulting analog signal is passed through a low-pass filter to a quadrature mixer for upconversion. The analog paths inadvertently introduce quadrature errors, referred to as IQ impairments 13a, which in particular cause differential errors between the inphase and quadrature components that vary as a function of frequency in the baseband. The upconverted signal 18 is output through the coupler 19, which is typically output to the input to the power amplifier for amplification in preparation for transmission from the antenna. The IQ pre-corrector 1a is controlled by the IQ correction controller 22, specifically through the control signals 23a in a manner that reduces the effects of the IQ impairment 13a.

Coupler 19 couples the sample of output 18 of upconverter 17 to input 20 of IQ direct conversion downconverter 21, which may be referred to as an observing receiver, and applies a sample thereto. The IQ downconverter 21 includes a quadrature mixer having analog in phase and quadrature paths as outputs through analog anti-aliasing filters and coupled to a pair of analog-to-digital converters (not shown). Separate analog paths unintentionally cause quadrature errors and the same for the transmit path, which is a problem especially if there is a differential error between the in-phase and quadrature components that varies as a function of frequency in the baseband .

The digital in phase and quadrature signal components 39i and 39q generated by the downconverter 21 are delivered to the IQ post corrector 1b in a manner that reduces the effects of the IQ impairment 13b in the receive path And is controlled by the IQ correction controller 22 via the signals 23b.

The IQ correction controller 22 compares the input signal components 2i and 2q with the signal components 25i and 25q output after the IQ post corrector 1b originating from the signals received by the receiving chain, . The IQ correction controller 22 controls the precoder 1a and the post corrector 1b via the signals 23a and 23b to generate an error between the input signal components 2i and 2q and the received signal components 25i and 25q Is minimized. The IQ controller, or possibly another controller (not shown), controls the relative phase between the local oscillator signal applied to the IQ up-converter 17 and the local oscillator signal applied to the IQ down-converter 21. In one embodiment, suitable control components are schematically illustrated as portions 37 and 38, the function of which is now described.

A signal generated by the local oscillator 37 and the IQ up-converter 17 and a signal generated by the local oscillator 37 and the IQ down-converter 21 are different between the two states Lt; / RTI > A comparison of the measurements made for each state allows the IQ pre-calibrator 1a to be corrected for IQ corruption in the up-converter 17 and the IQ post-calibrator 1b to be corrected for IQ corruption in the down-converter 21 . Typically, the signal output from the local oscillator 37 is separated so that one portion is supplied to the upconverter without phase shifting and the other portion is phase-shifted and supplied to the downconverter alternately by 0 or 90 degrees nominally. In principle, the phase shift does not need to be exactly 90 degrees, since any phase difference causes the system to resolve the necessary correction to the pre-corrector from the correction needed by the post corrector. It is desirable that this phase shift does not change the phase shift of the signal supplied to the upconverter because it is applied to the transmitted signal as unwanted phase modulation. Alternatively, a variable phase shift may be applied on the link from the coupler 19 to the input to the IQ downconverter 21. However, this requires that the phase shifter be wider than when it is placed in the local oscillator path, because the signal coupled by the coupler 19 can be modulated while the local oscillator signal is typically not.

Fig. 3 shows the structure of a frequency dependent quadrature correction network according to the invention, which can be used for the pre-correction network 1a or post-correction network Ib. The in-phase digital component enters 2i and the quadrature digital component enters 2q. The in-phase component is divided into two paths, one of which is transmitted to the combiner 8a through the digital filter 6a and from there to the in-phase output 3i. The other path is transmitted to the combiner 8b via the digital filter 6b and from there to the quadrature output 3q. The transmission characteristic of the digital filter 6a can be represented as 1 + A, indicating that the signal passes through unchanged substantially, except for a small factor A, which may be frequency dependent. For the filter 6b, the transmission characteristic may be represented as B, indicating that the signal is attenuated by a factor B that may be frequency dependent. Typically, A and B are much smaller than 1, preferably smaller than 0.1.

For the in-phase component, the quadrature component is divided into two paths, one through the digital filter 6d, to the combiner 8b, and from there to the quadrature output 3q. The other path is connected to the combiner 8a through the digital filter 6c and from there to the in-phase output component 3i. It can be shown that the transmission characteristic of the digital filter 6d is represented as 1 + D so that the signal passes substantially unchanged except for a small factor D, which may be frequency dependent. For filter 6c, the transmission characteristic may be denoted as C, indicating that the signal is attenuated by a factor C that may be frequency dependent. Typically, C and D are much smaller than 1, preferably smaller than 0.1.

Fig. 4 shows how the correction network used in the pre-correction network 1a in this example corrects the impairment 13. Fig. It can be seen that the damage is modeled as a network having a topology similar to the correction network 1a. The in-phase component 2i passes through the filter 6a and is multiplied by the transmission factor 1 + A and then passes through the impairment characteristic 14a which is multiplied by the transmission factor 1 + Ai. Where the term Ai is used to denote that Ai is a damage factor and not to indicate that it is an in-phase or imaginary element. It can be seen that the square expression is generated, and for small A, B, C and D squared values can be ignored.

4, in consideration of the damage, the in-phase signal component 3i entering the network simulating IQ damage 13 is multiplied by the factor (1 + Ai) It can be seen that the output of the rating IQ damage 13 is reached. The component of the quadrature signal component 3q is multiplied by Ci in the network simulated IQ impairment 13 and added to the in-phase component multiplied by the factor 1 + Ai in the further block 15a, Output 16i. ≪ / RTI >

In order to correct this damage to a first approximation, a correction network 1a is provided. The quadrature component 2q is multiplied by the factor C and added to the multiplied in phase component and the input to the network simulated IQ impairment 13 is multiplied by the factor C, (3i).

For small A and Ai, it can be seen that the damage factor Ai can be substantially eliminated if A = -Ai. Referring to FIG. 4, for example, in a two-phase phase signal path passing through blocks 6a and 14a, since the transmission factor will be (1 + A) (1-A) = 1-A ² , Able to know.

Similarly, for small C and Ci, the spurious quadrature component passing through the block 14c with the transmission factor Ci passes through the block 6c of the correction network 1a with the transmission factor C when C = Ci Is substantially canceled by component 2q.

If A, B, C, and D are less than 0.1, the squared value becomes less than 1% in terms of voltage, that is, -40 dB in terms of power.

Similarly, the in-phase component passing through B will cancel out substantially the spurious component Bi if B = -Bi and B is a small value, i. Also, if D = -Di, it can be seen that the damage of block 14d can also be canceled if D and C are small again.

It should be understood that the same principle will apply to the post-correction network following the corruption.

As already explained, the correction of differential errors within the transmission characteristics between the in-phase (I) and quadrature (Q) channels is particularly important. Thus, it is practically critical that the impairment causing the differential error be canceled, but it is acceptable to produce the same transmission characteristics on the I and Q channels, although not identical to the impairment-free transmission characteristics by combining impairments and corrections. That is, in the example of FIG. 4, the desired result need not be a situation where each of the I and Q channels has a transmission characteristic of one. It is also acceptable that I and Q channels have some different transmission characteristics when I and Q have the same characteristics for each. The operation of the control loop will automatically generate an optimal transmission characteristic for the filters in the correction network; It should be understood that the optimal solution does not have to be the application of the correction to simply return the transmission characteristics to the state that was present in the absence of quadrature impairment. Indeed, the operation of the control loop can potentially improve the operation of the system beyond simply eliminating the differential error between the in-phase and quadrature channels if the factor optimized by the control loop is changed in an efficient manner by improvement. For example, flattening the gain of the in-phase and quadrature channels can be achieved by operation of the control loop.

Fig. 5 shows the components of a typical digital filter 6a, 6b, 6c or 6d shown in Fig. The digital signal components 2i are transferred to a tapped delay line comprising a series of delay elements 9a and 9b which delay the signal components by time T, respectively; This delay time T may be a sampling period of the digital signals. After each delay element, a portion of the signal is tapped off and multiplied by a filter coefficient or weight Cn. The weighted components are then summed in the summing component 11 and delivered to the output 7a. The filter coefficients are shown as factors C1, C2, ... Cn. This structure constitutes a conventional finite impulse response (FIR) filter. The coefficients can be linear factors and can be controlled by the correction controller 22 to match the frequency response of the relevant component of the damage to optimally offset the damage. It is also possible that each tap has controllable coefficients that operate on a rectangular, cubic or other non-linear function of the tapped signal, in addition to the linear factor. This structure can be referred to as an expression of the Volterra series.

6 shows the IQ correction controller 22 in more detail. Phase 2i and quadrature (2q) components, which are inputs to the controller and are compared with the inphase 25i and quadrature 25q components from the observing receiver, are as described previously. In order to correct the phase shift and amplitude imbalance caused by the actual implementation of the system, it is necessary to perform a comparison to align the signal components from the input to the signal components from the observing receiver before generating the error signal. The reason for this is that the error signal should exhibit the contribution effect of IQ impairment rather than the effects due to other circuit elements. For control of the prior correction coefficients, the alignment and comparison block 26a operates in phase with the input signal components 2i, 2q to the received signals 25i, 25q. For control of the postcorrector coefficients, the alignment and comparison block 17b operates in phase with the input signal 2i, 2q to the received signal components 25i, 25q.

Considering first the operation of the controller 22 to update the pre-correction error coefficients, the alignment and comparison block 26a generates a reference output ref1 27a representing the input signal components, And generates an error output error 1 (28a) indicating the difference between the two. The signals ref1 and error1 are passed to the column error function block 29a. This block maintains the model of the corrector network in terms of topology and training involves adjusting the error coefficients so that the model of the corrector network generates an error signal when applied to the criterion. This can be done by conventional techniques. An appropriate technique generates a set of coefficients including the answer of a set of simultaneous equations for the input to the error signal. Typically this will be repeated several times and a least mean square method will be applied to generate optimal results from multiple measurements. A similar process is used to train the error coefficients for the post corrector using the training error count function block 29b.

As already mentioned, the described training process can not distinguish between the coefficients required for the pre-corrector and the coefficients required for the post-corrector; To solve this ambiguity, the training can be performed in two stages: first, using local oscillator signals for the upconverter and the downconverter in the first relative phase state, and then using the first relative phase state and typically 90 degrees Is performed using local oscillator signals of different second relative phase states.

The switch 30a operating on the output of the training error count function block 29a allows the error coefficients to be stored in the storage 31a for the local oscillator phase state 0 and the local oscillator Such that the error coefficients calculated in the phase state 90 (i.e., a state differing by 90 degrees from the phase state 0) are stored separately in the storage 31b. The sum of the two stores of error coefficients, schematically represented by part 32, is then used as an update to add to the dictionary corrector coefficients. The pre-corrector coefficients are updated iteratively by adding the sum of the stored error coefficients 32 trained in the two local oscillator states to substantially cancel the IQ impairment.

The postcorrection coefficients are updated in a similar procedure through portions 31c, 31d, and 33, but take the difference between them rather than the sum of the stored error coefficients for the two local oscillators. If the phase shift is introduced into the alignment process in one local oscillator phase relationship, but otherwise elsewhere, the corresponding phase shift must be applied to the stored error coefficients before the sum or difference operation to compensate for the phase shift. The combined process of phase shift and sum operation, and similarly the combined process of phase shift and phase difference operation, can be referred to as a vector combination.

Figure 7 shows the system of Figure 6 applied to a transmit chain in a power amplifier 40 that uses predistortion to correct for non-linearity.

It is particularly advantageous to use embodiments of the present invention with a system for linearizing the response of a power amplifier by predistortion. A wireless communication device, such as a base station and terminals, has a transmit chain that includes a power amplifier that amplifies the modulated signal to a high power level for transmission over a wireless channel. It can be seen that the elements in the transmit chain can introduce distortion into the transmitted signal and thus there have been several proposals for compensating for this distortion. One such proposal is a predistortion architecture in which the low power modulated signal is predistorted in a manner that compensates for the nonlinear effects of the power amplifier before being applied to the input of the power amplifier. The combination of the predistortion applied to the input signal and the (inevitable) nonlinear distortion applied to the input signal by the power amplifier produces a substantially distortion-free output signal.

Typically, an adaptive predistortion architecture applies predistortion to the digital domain before upconversion. The pre-distorted signals for the in-phase and quadrature channels are digitally generated in the baseband, separately converted to analog, and then converted to in-phase and quadrature branches of a direct conversion upconverter, also known as an IQ up- And directly upconverted. The portion of the upconverted output signal is fed back to the comparison function to control the predistortion system. This feedback path is known as an observing receiver and can be used to downconvert the sampled portion of the upconverted output signal to an intermediate frequency (IF), or to directly downconvert the sampled portion of the upconverted output signal to the baseband .

As mentioned, direct conversion techniques may be advantageous in terms of economical implementation, but may be affected by differential errors in the in-phase and quadrature signal paths. The direct conversion technique is based on the assumption that if the local oscillator for downconversion and upconversion operates at the same frequency and therefore a direct conversion architecture is used for the upconverter using the same synthesizer and an intermediate frequency architecture is used for the downconverter, There are certain advantages to avoiding risk.

However, the inherent quadrature errors prevent the use and effect of the direct conversion architecture of the observing receiver path. A method for correcting non-frequency dependent upconverter faults is known and involves the use of a conventional quadrature error corrector, as shown in Figure 1; However, this does not include the additional quadrature damage required to correct these damages. If the observing receiver uses a direct conversion architecture, quadrature errors will be introduced into the observing receiver. Although the quadrature errors in the upconverter have been compensated for once, errors in the downconverter impair the observation signal used to control the power amplifier predistortion and limit the effect of the amplifier predistortion correction loop. Therefore, it is necessary to correct errors generated by the up-converter and the down-converter. The system shown in Fig. 8 is designed to achieve this.

The power amplifier predistortion controller 44 receives the corrected signal components 25i and 25q from the observing receiver which are corrected by the input signal components 45i and 45q and the IQ post corrector 1b. The power amplifier predistortion controller 44 uses this input component to generate a predistortion characteristic to apply to the input signal in the PA predistortion block 43 to provide the input components 2i and 2q to the IQ pre- . The pre-corrected signal component is then applied to the IQ up-converter 17 and the upconverted signal component is passed through the coupler 19 to the power amplifier 40 and then through the second coupler 41 for transmission do. The switch 42 directs the signal components from the coupler 19 located upstream of the power amplifier (PA) 40 to the IQ downconverter 21 when the IQ correction controller is operating and the PA predistorter 44 , It directs the signal components from the coupler 41 to the downconverter 21. This means that the PA controller 44 is able to control the input of the PA predistortion block 43 to the transmit chains 45i and 45q and the output 25i of the IQ postcorner 1b The IQ controller correction controller 22 operates to minimize the difference between the outputs 25i and 25q of the IQ post corrector 1b with the appropriate setting of the switch 42, 2q) and the output of the IQ upconverter (17) to the IQ pre-calibrator (1a) (also measured in the IQ upconverter (1)).

Fig. 8 can operate without the post-correction, i.e. calculating the coefficients or applying to the post-corrector 1b, the system of Fig. FIG. 9 shows an IQ correction controller 22 that controls only the IQ pre-correction unit 1a. The pre-correction coefficients of the IQ pre-corrector 1a can be trained to offset the IQ impairment 13a in the up-converter even without the post corrector. In general, it is not necessary, but effective, to implement the post corrector 1b to increase the convergence speed of the IQ correction control loop. It is also desirable to correct the output of the IQ downconverter when used with a power amplifier predistortion control loop to optimize the performance of the loop.

10 illustrates a prior art pre-correction 4a and post-correction 4b network being controlled by the controller 60 based on optimization of anticipated attributes of the downconverted signal as disclosed in co-pending US patent application 11/962432 In the form of a block diagram. This application addresses the correction of non-frequency dependent quadrature errors in a system with a direct conversion upconverter and in an observing receiver using a direct conversion downconverter architecture. The measurements made for the upconverter and the downconverter local oscillator of the first phase relationship and the additional measurements made for the upconverter and downconverter local oscillators of the second phase relationship that are typically different from the first phase relationship by 90 degrees, A technique is disclosed that can distinguish between quadrature errors in the upconverter and quadrature errors in the downconverter. The measurement is made of the properties of the signals received at the observing receiver and is compared with the expected properties of the signal. For example, the long term correlation between the in-phase and quadrature components can be predicted to zero for an ideal signal such as a DC voltage component. The quadrature errors in the upconverter and downconverter paths are then corrected separately using a correction network that applies nominally the same correction regardless of the frequency within the baseband. Such corrections typically include voltage offset, i.e., correction of the DC offset, differential gain characteristics between the in-phase and quadrature signal paths, and phase errors between the in-phase and quadrature signal paths. The conventional quadrature correction network 4 can be used as shown in FIG.

However, there may be errors, especially differential errors, in upconversion and downconversion depending on the frequency in the baseband. For example, analogue filtering can introduce these errors into the anti-aliasing filter, in particular due to variations in the values of analog components and temperature in the component tolerance limits. The conventional correction network as shown in Fig. 1 can not correct this error.

In addition, measurements based on long-term averages of expected properties of received signals may not provide sufficient loop gain and stability to correct inherently slow and quadrature errors with high accuracy.

The prior art system of FIG. 10 can be used with the embodiments of the present invention already described, and in particular with the embodiments of the invention described in FIGS. 6 and 7, advantageously solving the correction of the frequency dependent quadrature errors described in FIG. Fig. 11 shows that the frequency dependent Ia and the conventional (4a) pre-correction network are connected in series and the frequency dependent Ib and the conventional (4b) post correct network are connected in series. To improve the operation of the frequency dependent predistorter control loop, it may be desirable to eliminate large errors using a conventional quadrature correction circuit as in FIG. A conventional pre-correction circuit can be controlled to optimize the predicted properties of the downconverted signal as previously described with reference to FIG. In particular, it is desirable to correct the DC offset in this manner, since a conventional quadrature correction circuit is suitable for this function and the control loop based on the expected properties of the observed signal is particularly effective for control of a conventional IQ correction circuit Do.

It will be appreciated that embodiments of the present invention may be applied to wired systems such as cable TV in addition to wireless systems.

The above embodiments should be understood as illustrative examples of the present invention. Any feature described in connection with any one embodiment may be used alone or in conjunction with other features described and may be used in conjunction with one or more features of any other embodiment, ≪ / RTI > Moreover, equivalents or modifications not described above may also be used without departing from the scope of the invention as defined in the appended claims.

Claims (14)

CLAIMS What is claimed is: 1. A method for controlling a transmission chain comprising a correction network, a quadrature up-converter and a quadrature down-converter, the correction network comprising a transmission characteristic of an in-phase signal path and a quadrature down- Wherein the quadrature up-converter is for use in up-converting an input signal to the correction network, wherein the quadrature up-converter is for use in up-converting an input signal to the correction network, The quadrature downconverter is for use in downconverting received signals, the upconverter having an output coupled to the downconverter, the correction network comprising a set of values of a filter tap coefficient, And wherein the input signal comprises input frequency components in a base band,
Coupling an output signal from an output of the upconverter to the quadrature downconverter;
Downconverting the combined output signal using the downconverter;
Comparing the downconverted output signal with the input signal; And
Applying a first correction to at least a first frequency component of the input frequency components, and modifying at least one of the values of the filter tap coefficients based on the comparing, correcting the frequency dependent quadrature error Wherein the first correction is dependent on the frequency of the at least first frequency component;
Generating a local oscillator signal by a local oscillator;
Providing, in a first operating state, the local oscillator signal to the upconverter and the downconverter;
Providing a local oscillator signal to one of the upconverter and the downconverter in a second operating state and providing a phase shifted version of the local oscillator signal to the other one of the upconverter and the downconverter;
Comparing the downconverted signal with the input signal for each operating state to determine an error signal;
Determining an intermediate set of values of the filter tap coefficients based on the error signal and the input signal for each operating state; And
Updating a current set of values of the filter tap coefficient based on a vector set of an intermediate set of values of the filter tap coefficient and a current set of values of a filter tap coefficient to generate an updated set of values of a filter tap coefficient
/ RTI > The method according to claim 1,
Determining an error signal from the downconverted signal and the input signal; And
Modifying at least one of the values of the filter tap coefficient using the error signal and the input signal
/ RTI > delete The method according to claim 1,
Wherein the downconverter is operative in association with a post-correction network configured to correct frequency dependent quadrature errors between in-phase and quadrature transmission paths in the quadrature downconverter, and wherein the downconverted signal is down Said post-correction network comprising configuration means for configuring values of a set of post-corrective filter tap coefficients and a set of said post-corrective filter tap coefficients,
The method comprises:
Updating the current values of the post corrector filter tap coefficients based on a vector combination of intermediate sets of values of the filter tap coefficients and the current set of post corrector coefficients to generate an updated set of post corrector coefficients; And
By applying a second correction to at least a second frequency component of the downconverted frequency components by controlling the post correction network with a updated set of the post corrector values, Steps to correct for quadrature errors
Further comprising:
Wherein the second correction is dependent on the frequency of the at least second frequency component. A computer readable medium encoded with computer readable code for causing a controller to perform the method of claim 1 or claim 2. A controller configured to perform the method of claim 1 or 2. As a transmission chain,
A correction network for correcting frequency dependent quadrature errors between in-phase and quadrature signal paths for transmission of input signals having frequency components, said correction network comprising an in-phase input port, a quadrature input port, an in- Wherein each input port is connected to a respective output port by a digital filter network and wherein the digital filter network comprises configuration means for configuring values of a set of filter tap coefficients and a set of filter tap coefficients, Included -; And
A quadrature up converter for upconverting the input signal;
A transmission path including a transmission path;
A coupler for receiving a portion of the upconverted input signal; And
A quadrature downconverter for downconverting a portion of the upconverted input signal received by the coupler;
; And
A local oscillator operable in a first operating state and a second operating state, wherein the local oscillator generates a local oscillator signal and provides the local oscillator signal to the upconverter, and when operating in the first operating state, To provide a down-converter with a phase-shifted version of the local oscillator signal when operating in the second operating state;
Comparing the downconverted signal with the input signal to determine an error signal;
Modify values of the set of filter tap coefficients based on the error signal and the input signal;
To correct the frequency dependent quadrature error by applying a correction to at least one of the frequency components by controlling the correction network with a set of updated values
The controller
/ RTI > A wireless communication device comprising a transmission chain according to claim 7,
Wherein the wireless communication device is a wireless terminal. A wireless communication device comprising a transmission chain according to claim 7,
Wherein the wireless communication device is a wireless base station. A correction network for correcting a frequency dependent quadrature error between a transmission characteristic of an in-phase signal path and a transmission characteristic of a quadrature signal path,
Wherein the in-phase signal path is for transmission of in-phase portions of a signal having frequency components, the quadrature signal path for transmission of quadrature portions of the signal, the correction network comprising an in-phase input port, An input port, an in-phase output port, and a quadrature output port,
Each input port connected to a respective output port by a digital filter network, said digital filter network comprising configuration means for configuring values of a set of filter tap coefficients and a set of said filter tap coefficients. 11. The method of claim 10,
The digital filter network comprising:
A first digital filter coupling the in-phase input port to the in-phase output port;
A second digital filter coupling the in-phase input port to the quadrature output port;
A third digital filter coupling the quadrature input port to the inphase output port; And
A fourth digital filter coupling the quadrature input port to the quadrature output port,
Lt; / RTI >
Each digital filter comprising respective configuration means for constructing a subset of each of the filter tap coefficients and a respective subset of the respective filter tap coefficients in the set of filter tap coefficients. 11. The method of claim 10,
Wherein the at least one digital filter of the digital filter network comprises a finite impulse response filter. 11. The method of claim 10,
Wherein at least one of the digital filters of the digital filter network comprises an infinite impulse response filter. 11. The method of claim 10,
Wherein the at least one digital filter of the digital filter network comprises a polynomial structure based on the Volterra series.

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